The Terebratulidae show the “loop type” of brachial apparatus. In Waldheimia (Fig. [329]), which may be taken as an example, we notice first in the dorsal valve the “crura” (cr), from which arise the two “descending branches” which run forwards and then are bent back to form the “ascending branches” which are united by the “transverse band.” In some genera the “ascending branches” may be reduced to mere points, and the “transverse band” become a median vertical plate; the “crura,” too, may be fused so as to form a “crural band”; and the “descending branches” may be connected by a cross band—the “jugal band.” In Stringocephalus (Fig. [326], l, s.p) the loop is furnished on its inner edge with radiating processes; and in Argiope the loop is simple, not reflected, and fused with marginal septa; while in the Thecidiidae it is more or less fused with the shell itself, and with the mass of calcareous spicules secreted by the mantle.
The “spiral-cone type” of brachial apparatus is found in the Spiriferidae, Atrypidae, and Koninckinidae, and consists of two spirally-enrolled calcified lamellae, forming two cones with their apices directed laterally (Spirifera, Fig. [330]), or towards the interior of the dorsal valve (Atrypa, Fig. [332]), or towards each other (Glassia); or forming two flat spirals in the same plane (Koninckinidae). A “jugal band” is generally present, but varies much in position, and in some genera has complicated posterior processes.
The Rhynchonellidae have no loop or spiral cones, but merely a pair of short “crura.”
Fig. 330.—Spirifera striata. (Carboniferous Limestone.) Showing brachial spires.
The principal modifications in the attachments of the muscles in the Testicardines are illustrated by Productus giganteus (Fig. [333]), Leptaena rhomboidalis (Fig. [334]), and Waldheimia flavescens (Fig. [329]).
In Productus (Fig. [333]) we see in the ventral valve a pair of dendritic occlusor, often called adductor, impressions and a pair of large flabellate divaricator impressions. In the dorsal valve the large “cardinal process” served for the attachment of the divaricator, and a low median septum separated the dendritic occlusor scars, which are rarely divisible into anterior and posterior pairs.
Fig. 331.—Atrypa reticularis. (Wenlock Limestone.)
